Is an Adductor Canal Block Really Better Than a Femoral Block for Anterior Cruciate Ligament Reconstruction?

We read with interest the study conducted by Abdallah et al.¹  comparing single femoral and adductor canal (AC) blocks on quadriceps function in patients undergoing anterior cruciate ligament (ACL) reconstruction. Although their design was complex, the authors seem to have not considered some fundamental principles of peripheral nerve block research. This unfortunately may limit the clinical usefulness of the authors’ findings.

1. The femoral nerve is a sensory and motor nerve, whereas the saphenous nerve (nerve blocked by injecting local anesthetics in AC) is only sensory. In these conditions, it is not surprising that a femoral block using an anesthetic rather than analgesic concentration of 0.5% ropivacaine would produce a motor block (femoral but not AC block).

2. AC space is a very small space, which is too small to accommodate a volume of 20 ml. In these conditions what was the rationale for choosing a 20-ml volume versus 10 ml or less?^2,3 

3. For the past 20 yr, the concentration and volume of local anesthetics used for perioperative analgesia have decreased in order to minimize any motor block when performing a femoral block. Nowadays, the use of 0.2% or even 0.1% ropivacaine and a volume of 10 ml rather than 20 ml is recommended. Therefore, it would have been much more clinically relevant to use 10 ml of 0.1% or 0.2% ropivacaine to assess the relative benefit of each approach.^4,5 

4. Assessing motor function after a block would be more clinically relevant if done after, rather than before, surgery.

5. Patients undergoing ACL are usually young. Since the duration of a block has been established to vary with age, shorter in young versus older adults,⁶  under these conditions, it is likely that most patients could have recovered from the block within 12 h.²  Are the effects reported at 24 h a reflection of those observed at 12 h?

Finally, it was established 20 yr ago that the clinical benefits of a femoral block performed for knee surgery varies with the type of knee surgery and expected associated pain.⁷  This is especially important when considering (as appropriately acknowledged by the authors) that only two thirds of the patients included in this study underwent an ACL repair using a hamstring tendon (that being the type of ACL repair demonstrated to really benefit from a femoral nerve block).

In conclusion, in the absence of data obtained using a low concentration and volume of ropivacaine in patients undergoing ACL using a hamstring tendon, it is not clear which block (femoral or AC block) is the most appropriate to optimize the surgical outcome.

The authors declare no competing interests.